﻿464 
  MR 
  WILLIAM 
  SWAN 
  ON 
  THE 
  

  

  trusted, 
  of 
  very 
  much 
  greater 
  extent 
  ;"* 
  and 
  yet 
  spots 
  of 
  such 
  immense 
  magnitude 
  

   seldom 
  last 
  much 
  longer 
  than 
  six 
  weeks, 
  so 
  that 
  their 
  edges 
  must 
  often 
  approach 
  

   at 
  the 
  rate 
  of 
  1000 
  miles 
  a-day.f 
  

  

  Now, 
  as 
  the 
  spots 
  have 
  been 
  supposed 
  to 
  arise 
  from 
  upward 
  currents 
  causing 
  

   apertures 
  in 
  the 
  sun's 
  luminous 
  atmosphere, 
  I 
  conceive 
  the 
  higher 
  red 
  prominences, 
  

   or 
  those 
  which 
  remain 
  visible 
  at 
  the 
  middle 
  of 
  the 
  total 
  phase 
  of 
  a 
  central 
  eclipse, 
  

   may 
  in 
  like 
  manner 
  be 
  formed 
  (as 
  represented 
  at/and 
  g, 
  fig. 
  10) 
  by 
  the 
  same, 
  or 
  

   similar 
  currents, 
  in 
  the 
  sun's 
  atmosphere, 
  breaking 
  through 
  the 
  envelope 
  of 
  cloud 
  

   that 
  surrounds 
  him, 
  bending 
  back 
  the 
  edges 
  of 
  the 
  apertures 
  they 
  have 
  formed, 
  

   and 
  sometimes 
  carrying 
  up 
  detached 
  masses 
  of 
  cloud, 
  such 
  as 
  that 
  which 
  was 
  seen 
  

   at 
  the 
  late 
  eclipse. 
  We 
  may, 
  however, 
  suppose 
  the 
  envelope 
  of 
  cloud 
  to 
  be 
  some- 
  

   times 
  simply 
  raised 
  (as 
  at 
  h), 
  without 
  being 
  broken 
  through 
  ; 
  and 
  in 
  that 
  state 
  it 
  

   may 
  form 
  the 
  conical 
  prominences 
  which 
  were 
  observed 
  at 
  the 
  late 
  eclipse. 
  

  

  Since 
  the 
  prominences 
  reflect, 
  they 
  must 
  also 
  absorb 
  light 
  ;■$ 
  and 
  thus, 
  the 
  hy- 
  

   pothesis 
  which 
  has 
  been 
  proposed 
  regarding 
  them, 
  assumes 
  the 
  presence 
  of 
  an 
  en- 
  

   velope 
  of 
  cloud 
  surrounding 
  the 
  sun's 
  luminous 
  atmosphere, 
  capable 
  of 
  absorbing 
  part 
  

   of 
  his 
  light, 
  and 
  subject 
  to 
  occasional 
  interruptions 
  of 
  its 
  continuity. 
  

  

  If, 
  then, 
  such 
  an 
  envelope 
  surrounds 
  the 
  sun, 
  it 
  will 
  probably 
  be 
  connected 
  

   with 
  various 
  solar 
  phenomena. 
  Let 
  us 
  now 
  inquire 
  whether 
  any 
  appearances 
  

   presented 
  by 
  the 
  sun 
  afford 
  additional 
  evidence 
  of 
  its 
  existence. 
  

  

  2. 
  On 
  the 
  Increased 
  Brightness 
  of 
  the 
  Corona 
  in 
  the 
  neighbourhood 
  of 
  the 
  Bed 
  Prominences. 
  

  

  According 
  to 
  my 
  observations 
  of 
  the 
  late 
  eclipse, 
  the 
  hook-shaped 
  prominence 
  

   was 
  accompanied 
  by 
  increased 
  brightness 
  of 
  the 
  corona 
  in 
  its 
  neighbourhood. 
  

   (Plate 
  XIL). 
  Now 
  this 
  is 
  a 
  necessary 
  consequence 
  of 
  the 
  supposition, 
  that 
  the 
  higher 
  

   red 
  prominences 
  are 
  the 
  upturned 
  edges 
  of 
  apertures 
  in 
  the 
  envelope 
  of 
  cloud 
  sur- 
  

   rounding 
  the 
  sun 
  ; 
  for 
  the 
  absorbent 
  medium 
  having 
  been 
  removed 
  in 
  forming 
  the 
  

   apertures, 
  the 
  sun's 
  light 
  ought 
  to 
  illuminate 
  the 
  corona 
  more 
  powerfully 
  over 
  an 
  

   aperture 
  than 
  elsewhere. 
  At 
  the 
  same 
  time, 
  it 
  does 
  not 
  follow 
  that 
  all 
  red 
  pro- 
  

   minences 
  should 
  be 
  near 
  bright 
  portions 
  of 
  the 
  corona 
  ; 
  for 
  a 
  prominence 
  may 
  be 
  

   formed 
  by 
  the 
  cloudy 
  stratum 
  being 
  simply 
  raised, 
  without 
  being 
  perforated. 
  

  

  * 
  Dr 
  Wilson 
  of 
  Glasgow, 
  who, 
  by 
  observing 
  a 
  large 
  solar 
  spot, 
  was 
  led 
  to 
  the 
  discovery 
  that 
  it 
  

   was 
  an 
  aperture 
  in 
  the 
  sun's 
  luminous 
  atmosphere, 
  estimates 
  the 
  depth 
  of 
  its 
  nucleus 
  as 
  " 
  not 
  less 
  

   than 
  a 
  semidiameter 
  of 
  the 
  earth 
  below 
  the 
  level 
  of 
  the 
  sun's 
  spherical 
  surface." 
  See 
  his 
  highly 
  in- 
  

   teresting 
  paper, 
  Philosophical 
  Transactions, 
  vol. 
  lxiv., 
  1774. 
  

  

  f 
  Herschel's 
  Outlines 
  of 
  Astronomy, 
  1851, 
  par. 
  386. 
  

  

  + 
  In 
  certain 
  circumstances 
  steam 
  has 
  a 
  red 
  colour. 
  May 
  not 
  the 
  rose 
  colour 
  of 
  the 
  prominences 
  

   indicate 
  a 
  property 
  of 
  the 
  vapour 
  composing 
  them 
  analogous 
  to 
  that 
  possessed 
  by 
  steam, 
  or, 
  if 
  they 
  

   consist 
  of 
  aqueous 
  vapour, 
  identical 
  with 
  it 
  ? 
  Professor 
  Forbes, 
  to 
  whom 
  we 
  are 
  indebted 
  for 
  our 
  

   knowledge 
  of 
  the 
  red 
  colour 
  of 
  steam, 
  regards 
  it 
  as 
  the 
  principal 
  or 
  only 
  cause 
  of 
  the 
  rosy 
  tint 
  

   observed 
  in 
  clouds. 
  — 
  See 
  his 
  interesting 
  paper, 
  Edinburgh 
  Transactions, 
  vol. 
  xiv., 
  p. 
  371. 
  

  

  